Try to leave out one of the middle sheaves (rollers) in your sails tack fitting.
I just rigged a Severne NCX 7.5m2 the other day, and I had no problems by using the outside pulleys (sheaves) and skipping one of the inside (the 2nd) pulleys.
If you (and Mim) are not using a downhauling tool (crank type works best for me) you really might want to consider the damage you may be doing to your lower back by trying to pull all that tension with your lower back muscles.
Put all the load on an easy to use tool and save your back.
The tool makes "micro adjusting" much easier as well.
Hope this helps,